I suppose if you're stuck with only one IP and don't have two gateway IPs you can use, you can create ns1.yourdomain.com and ns2.yourdomain.com and have them both redirect to your only IP. (You have to create both as users will need a primary and secondary nameserver for registration/propogation even if they lead to the same IP.) And you would do all of this at your domain name registar in the host names or nameserver section.